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Hildegard Meyer-Ortmanns

Publications and source records attributed to Hildegard Meyer-Ortmanns.

45 records · Page 3Linked to original sources
arXiv2004

Dynamics of Phase Transitions: The 3D 3-state Potts model

In studies of the QCD deconfining phase transition or cross-over by means of heavy ion experiments, one ought to be concerned about non-equilibrium effects due to heating and cooling of the system. In this paper we extend our previous study of Glauber dynamics of 2D Potts models to the 3D 3-state Potts model, which serves as an effective model for some QCD properties. We investigate the linear theory of spinodal decomposition in some detail. It describes the early time evolution of the 3D model under a quench from the disordered into the ordered phase well, but fails in 2D. Further, the quench leads to competing vacuum domains, which are difficult to equilibrate, even in the presence of a small external magnetic field. From our hysteresis study we find, as before, a dynamics dominated by spinodal decomposition. There is evidence that some effects survive in the case of a cross-over. But the infinite volume extrapolation is difficult to control, even with lattices as large as $120^3$.

hep-lat↗
arXiv2003

Functional Complexity Measure for Networks

We propose a complexity measure which addresses the functional flexibility of networks. It is conjectured that the functional flexibility is reflected in the topological diversity of the assigned graphs, resulting from a resolution of their vertices and a rewiring of their edges under certain constraints. The application will be a classification of networks in artificial or biological systems, where functionality plays a central role.

cond-mat↗
arXiv2003

Dynamics of Phase Transitions by Hysteresis Methods I

In studies of the QCD deconfining phase transition or crossover by means of heavy ion experiments, one ought to be concerned about non-equilibrium effects due to heating and cooling of the system. Motivated by this, we look at hysteresis methods to study the dynamics of phase transitions. Our systems are temperature driven through the phase transition using updating procedures in the Glauber universality class. Hysteresis calculations are presented for a number of observables, including the (internal) energy, properties of Fortuin-Kasteleyn clusters and structure functions. We test the methods for 2d Potts models, which provide a rich collection of phase transitions with a number of rigorously known properties. Comparing with equilibrium configurations we find a scenario where the dynamics of the transition leads to a spinodal decomposition which dominates the statistical properties of the configurations. One may expect an enhancement of low energy gluon production due to spinodal decomposition of the Polyakov loops, if such a scenario is realized by nature.

hep-lat↗
arXiv2002

Immigration, integration and ghetto formation

We study ghetto formation in a population with natives and immigrants in the framework of the two-dimensional Ising-model with Kawasaki-exchange dynamics. It is the phase structure of the Ising model, the integration speed and the immigration rate which determine whether ghetto formation between natives and immigrants can be avoided or not. Our simulations are performed in and out-of equilibrium.

cond-mat↗
arXiv2001

Catastrophic senescence of the Pacific salmon without mutation-accumulation

We derive catastrophic senescence of the Pacific salmon from an aging model which was recently proposed by Stauffer. The model is based on the postulates of a minimum reproduction age and a maximal genetic lifespan. It allows for self-organization of a typical age of first reproduction and a typical age of death. Our Monte Carlo simulations of the population dynamics show that the model leads to catastrophic senescence for semelparous reproduction as it occurs in the case of salmon, but to a more gradually increase of senescence for iteroparous reproduction.

cond-mat↗
arXiv1996

Computer Aided Series Expansions for Critical Phenomena

Under quite general conditions critical phenomena can be described with high order linked cluster expansions. The coefficients of the series admit a graphical expansion that is generated with the aid of computers. Our generalization of linked cluster expansions from an infinite to a finite volume allows to perform a finite size scaling analysis. We also indicate a generalization to Dynamical Linked Cluster Expansions with possible applications to spin glasses and neural networks with coupled spin and interaction dynamics.

hep-lat↗
arXiv1996

Phase transitions in quantum chromodynamics

The current understanding of finite temperature phase transitions in QCD is reviewed. A critical discussion of refined phase transition criteria in numerical lattice simulations and of analytical tools going beyond the mean-field level in effective continuum models for QCD is presented. Theoretical predictions about the order of the transitions are compared with possible experimental manifestations in heavy-ion collisions. Various places in phenomenological descriptions are pointed out, where more reliable data for QCD's equation of state would help in selecting the most realistic scenario among those proposed. Unanswered questions are raised about the relevance of calculations which assume thermodynamic equilibrium. Promising new approaches to implement nonequilibrium aspects in the thermodynamics of heavy-ion collisions are described.

hep-lat↗
arXiv1994

How sharp is the chiral crossover phenomenon for realistic meson masses?

The mass dependence of the chiral phase transition is studied in the linear $SU(3)\times SU(3)$ sigma-model to leading order in a $1/N_f$-expansion, $N_f$ denoting the number of flavours. For realistic meson masses we find a smooth crossover between $T\sim181.5$ to 192.6~[MeV]. The crossover looks more rapid in the light quark condensate than in thermodynamic quantities like the energy and entropy densities. The change in the light quark condensate in this temperature interval is $\sim$~50\% of the zero-temperature condensate value, while the entropy density increases by ($5.5\pm0.8)\cdot10^{-3}$~[GeV$^3$]. Since the numerical error is particularly large in this region, we cannot rule out a finite latent heat smaller than 0.2~[GeV/fm$^3$]. The chiral transition is washed out for an average pseudoscalar meson octet mass of 203~[MeV]. This gives an upper bound on the first-order transition region in the meson mass parameter space. The corresponding ratio of critical to realistic light current quark masses $m^{crit}_{u,d}/m_{u,d}$ is estimated as $0.26\pm0.08$. This result is by an order of magnitude larger than the corresponding mean-field value. Therefore the

hep-ph↗